(1) Field of the Invention
The present invention generally relates to an altitude determining apparatus for an automotive vehicle, and more particularly to an apparatus for determining an altitude condition of an automotive vehicle by comparing an intake air flow rate with a reference flow rate obtained from an engine speed and a throttle angle.
(2) Description of the Related Art
When an automotive vehicle is running in a high altitude condition, an intake air flow rate of the automotive vehicle in response to a throttle angle of a throttle valve becomes smaller than an intake air flow rate in response to the same throttle angle in a low altitude condition since the atmospheric pressure and air density at high altitudes are lower than the atmospheric pressure and air density at low altitudes or the sea level. Thus, when the vehicle is running in a high altitude condition, the output power of the engine becomes lower than a corresponding output power of the engine in a low altitude condition if the throttle valve is set to the same throttle angle. It is necessary to determine an altitude condition of the vehicle in order to suitably control the running of the vehicle.
Japanese Laid-Open Patent Publication No.3-185250 discloses a device for determining an altitude condition of an automotive vehicle. In this device, an intake air flow rate obtained from an air flow meter during the running of the vehicle is compared with a reference flow rate obtained from a sensed engine speed and throttle angle, and an altitude condition of the vehicle is determined based on the result of the comparison. In the above mentioned device, the reference flow rate is read in response to the sensed engine speed and throttle angle from a memory in which a map defining a predetermined characteristic of reference flow rate in the relationship between the engine speed and the throttle angle is stored.
However, in a case of an automotive vehicle having an exhaust gas recirculation system (hereinafter called an EGR system), a certain amount of exhaust gas is fed back to an intake passage of the engine when the engine is subjected to exhaust gas recirculation by the EGR system. As the exhaust gas is mixed with a mixture of intake air and fuel vapor before the mixture is supplied to a combustion chamber of the engine, the combustion temperature is lowered due to the recirculated exhaust gas, in order to reduce the amount of nitrogen oxides (NOx) produced from the engine.
In the vehicle of the type described above, a difference in pressure between an upstream location of the throttle valve in the intake passage and at a downstream location thereof when the EGR is performed becomes smaller, due to the recirculated exhaust gas, than the pressure difference when no EGR is performed. Thus, when the engine is subjected to the EGR, the intake air flow rate becomes smaller than when no EGR is performed if the throttle valve is set to the same throttle angle.
In addition, in a case of an automotive vehicle having an evaporated fuel purge system (hereinafter called an EFP system), a certain amount of fuel vapor is fed into the intake passage of the engine when the engine is operating under prescribed operating conditions. In order to prevent evaporated fuel of a fuel tank from escaping to the atmosphere, fuel vapor of the fuel tank in the EFP system is absorbed in a canister, and the absorbed fuel vapor is fed from the canister into the intake passage due to negative pressure produced in the intake passage. Thus, when the engine is subjected to the evaporated fuel purge (EFP), the intake air flow rate becomes smaller than when no EFP is performed if the throttle valve is set to the same throttle angle.
FIG. 12 schematically shows several characteristics of intake air flow rate GNAFM in response to a throttle angle TA when the vehicle is running under different operating conditions. In FIG. 12, the intake air flow rate GNAFM is measured based on an output voltage of an air flow meter, and the throttle angle TA is measured from a throttle valve. A characteristic of intake air flow rate GNAFM in response to the throttle angle TA when the vehicle is running in a low altitude condition (neither EGR nor EFP performed) is indicated by a one-dot chain line I in FIG. 12. A characteristic of intake air flow rate GNAFM in response to the throttle angle TA when the vehicle is running in a high altitude condition (neither EGR nor EFP performed) is indicated by a two-dot chain line II in FIG. 12. As the air density at high altitudes is lower than the air density at low altitudes, the intake air flow rate GNAFM indicated by the line II (the high altitude condition) is lower than the intake air flow rate GNAFM indicated by the line I (the low altitude condition). Therefore, if the throttle valve is set to the same throttle angle TA, the output power of the engine in the high altitude condition is lower than the output power of the engine in the low altitude condition. During the running of the vehicle in the high altitude condition, if an accelerator pedal is depressed by a vehicle operator to set the throttle valve to a greater throttle angle in order to increase the engine power to a greater power that is the same as that in the low altitude condition, the intake air flow rate GNAFM (indicated by the line II in FIG. 12) is maintained at a constant level and it does not reach the intake air flow rate in the low altitude condition.
In FIG. 12, a solid line III indicates a characteristic of intake air flow rate GNAFM in response to the throttle angle TA when the vehicle is running in the low altitude condition while the EFP is performed, and a dotted line IV indicates a characteristic of intake air flow rate GNAFM in response to the throttle angle TA when the vehicle is running in the low altitude condition while the EGR is performed. Both the intake air flow rates GNAFM indicated by the lines III and IV are lower than the intake air flow rate GNAFM indicated by the line I if the throttle valve is set to the same throttle angle TA. The intake air flow rate measured at the air flow meter when either the EGR or the EFP is performed to supply additive gas to the intake passage of the engine is lower than when neither the EGR nor the EFP is performed if the amounts of air supplied to the combustion chamber of the engine in both cases are equal to each other.
However, when the above conventional device is applied to the automotive vehicles of the types described above, it is impossible to detect whether the intake air flow rate in response to the throttle angle has been lowered due to a high altitude condition or due to the performing of the EFP or EGR. Since the altitude condition of the vehicle is merely determined based on the sensed engine speed and throttle angle, there is a problem in that an erroneous altitude condition may be determined by the above conventional device.